1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, and a facsimile apparatus, which uses an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
Recently, full color machines capable of outputting a plurality of colors have become the mainstream of electrophotographic and electrostatic recording image forming apparatuses. For example, many tandem type image forming apparatuses, in which a plurality of image bearing members of different developing colors is arranged along a rotation path of an intermediate transfer member to form a full color image, have been put to practical use.
Generally speaking, in electrophotographic and electrostatic recording image forming apparatuses, the electric resistances of an intermediate transfer member and transfer rollers and/or the thicknesses of surface layers of photosensitive members may change with a change in an atmospheric environment such as temperature and humidity, or with the use of the apparatuses. Thus, according to such changes, it is desirable to change a transfer voltage to be applied to a transfer member. To obtain a desired transfer voltage during an image forming operation, a voltage determination control for determining a control value (voltage value) for constant voltage control is performed before the image forming operation. For example, Japanese Patent Application Laid-Open No. 2-123385 discusses active transfer voltage control (ATVC) as a voltage determination method. In the ATVC, a desired constant current voltage is applied to a photosensitive member from a transfer roller during a non-image forming operation of the image forming apparatus, and the value of the voltage applied at that time is stored. The electric resistance of the transfer member is thereby detected, and a constant voltage according to the electric resistance value is applied to the transfer roller as a transfer voltage at the time of transfer in the image-forming operation.
Further, it is desirable that an image forming apparatus that performs continuous image formation change the transfer voltage applied to a transfer member, according to a change in the atmospheric environment inside the apparatus due to the continuous operation. Thus, sheet-to-sheet correction control may be performed to correct a primary transfer voltage value. The sheet-to-sheet correction control includes monitoring a primary transfer current at a timing corresponding to an interval between transfer materials during the continuous image formation (hereinafter also referred to as “an interval between sheets”), and increasing or decreasing the applied voltage by a certain value if a difference equal to or greater than a certain current amount occurs with respect to a target current.
Japanese Patent Application Laid-Open No. 2008-129471 discusses a method for enabling optimum constant voltage settings even under extreme conditions when the present electric resistance of a transfer member changes greatly due to a temporal change or temperature variations. More specifically, a target transfer current value is set based on a predetermined table according to the electric resistance of the transfer member obtained during the foregoing ATVC. The table contains values that are set in advance to uniquely reduce the target transfer current value as the electric resistance increases.
However, the foregoing conventional method by which the target transfer current value is determined according to the electric resistance obtained in the voltage determination control performed before the image formation may fail to apply an appropriate transfer voltage during the image formation.
In particular, if an intermediate transfer member of which the electric resistance changes greatly during the image formation is used, the optimum current varies during the image formation. As a result, the current value determined according to the electric resistance obtained before the image formation deviates gradually from the optimum current if a large amount of images are formed.
Furthermore, if the foregoing conventional control of uniquely reducing the target transfer current value with an increase in the electric current is used with an intermediate transfer member of which the electric resistance changes greatly during the image formation, the following problem may occur. If the electric resistance of the intermediate transfer member is low, the target transfer current value becomes high and an excessive current may flow through the photosensitive member, thereby causing a memory in the photosensitive member. On the other hand, if the electric resistance of the intermediate transfer member is high, the target transfer current value may be lowered too much, thereby causing a transfer defect.